Immature lung TNFR2- conventional DC 2 subpopulation activates moDCs to promote cyclic di-GMP mucosal adjuvant responses in vivo

Targeting dendritic cell-specific TNFR2 improves skin and joint inflammation by inhibiting IL-12/ IFN-γ pathways in a mouse model of psoriatic arthritis

Psoriasis (PsO) and Psoriatic arthritis (PsA) are immune-mediated inflammatory diseases affecting the skin and joints. Approximately, 30% of patients with PsO develop PsA over time with both conditions being associated with elevated tumor necrosis factor-alpha (TNF-α) expression. TNF-α mediates its effect through two membrane receptors, TNFR1 and TNFR2. While current TNF-α-neutralizing agents, targeting both TNFR1 and TNFR2 receptors, constitute the primary treatment for psoriatic diseases, their long-term use is limited due to an increase in opportunistic infections, tuberculosis reactivation and malignancies likely attributed to TNFR1 inactivation. Recent findings suggest a pivotal role of TNFR2 in psoriatic disease, as evidenced by its amelioration in global TNFR2-knockout (TNFR2KO) mice, but not in TNFR1KO mice. The diminished disease phenotype in TNFR2KO mice is accompanied by a decrease in DC populations. However, the specific contribution of TNFR2 in dendritic cells (DCs) remains unclear. Here, utilizing a mannan-oligosaccharide (MOS)-induced PsA model, we demonstrate a significant reduction in PsA-like skin scaling and joint inflammation in dendritic cell-specific TNFR2 knockout mice (DC-TNFR2KO). Notably, MOS treatment in control mice (TNFR2 fl/fl) led to an increase in conventional type 1 dendritic cells (cDC1) population in the spleen, a response inhibited in DC-TNFR2KO mice. Furthermore, DC-TNFR2KO mice exhibited reduced levels of interleukin-12 (IL-12), a Th1 cell activator, as well as diminished Th1 cells, and interferon-gamma (IFN-γ) levels in the serum compared to controls following MOS stimulation. In summary, our study provides compelling evidence supporting the role of TNFR2 in promoting PsA-like inflammation through cDC1/Th1 activation pathways.

Signaling pathways involved in the biological functions of dendritic cells and their implications for disease treatment

The ability of dendritic cells (DCs) to initiate and regulate adaptive immune responses is fundamental for maintaining immune homeostasis upon exposure to self or foreign antigens. The immune regulatory function of DCs is strictly controlled by their distribution as well as by cytokines, chemokines, and transcriptional programming. These factors work in conjunction to determine whether DCs exert an immunosuppressive or immune-activating function. Therefore, understanding the molecular signals involved in DC-dependent immunoregulation is crucial in providing insight into the generation of organismal immunity and revealing potential clinical applications of DCs. Considering the many breakthroughs in DC research in recent years, in this review we focused on three basic lines of research directly related to the biological functions of DCs and summarized new immunotherapeutic strategies involving DCs. First, we reviewed recent findings on DC subsets and identified lineage-restricted transcription factors that guide the development of different DC subsets. Second, we discussed the recognition and processing of antigens by DCs through pattern recognition receptors, endogenous/exogenous pathways, and the presentation of antigens through peptide/major histocompatibility complexes. Third, we reviewed how interactions between DCs and T cells coordinate immune homeostasis in vivo via multiple pathways. Finally, we summarized the application of DC-based immunotherapy for autoimmune diseases and tumors and highlighted potential research prospects for immunotherapy that targets DCs. This review provides a useful resource to better understand the immunomodulatory signals involved in different subsets of DCs and the manipulation of these immune signals can facilitate DC-based immunotherapy.

Putrescine accelerates the differentiation of bone marrow derived dendritic cells via inhibiting phosphorylation of STAT3 at Tyr705

Dendritic cells (DCs) play pivotal roles in immune responses. The differentiation and function of DCs are regulated by environmental metabolites. Putrescine is ubiquitous in various metabolic microenvironments and its immunoregulation has been of increasing interest. However, the mechanisms associated with its DC-induced immunoregulation remain unclear. In this study, we found putrescine promoted induction of immature bone marrow derived DCs (BMDCs), along with the increased phagocytosis and migration, and altered cytokine secretion in immature BMDCs. Transcriptomic profiles indicated significantly impaired inflammatory-related pathways, elevated oxidative phosphorylation, and decreased p-STAT3 (Tyr705) expression. Additionally, putrescine performed minor influence on the lipopolysaccharide (LPS)-induced maturation of BMDCs but significantly impaired LPS-induced DC-elicited allogeneic T-cell proliferation as well as the cytokine secretion. Furthermore, molecular docking and dynamics on the conjugation between putrescine and STAT3 revealed that putrescine could be stably bound to the hydrophilic cavity in STAT3 and performed significant influence on the Tyr705 phosphorylation. CUT&Tag analysis uncovered altered motifs, downregulated IFN-γ response, and upregulated p53 pathway in Putrescine group compared with Control group. In summary, our results demonstrated for the first time that putrescine might accelerate the differentiation of BMDCs by inhibiting the phosphorylation of STAT3 at Tyr705. Given that both DCs and putrescine have ubiquitous and distinct roles in various immune responses and pathogeneses, our findings may provide more insights into polyamine immunoregulation on DCs, as well as distinct strategies in the clinical utilization of DCs by targeting polyamines.

Antigen Presentation in the Lung

The lungs are constantly exposed to environmental and infectious agents such as dust, viruses, fungi, and bacteria that invade the lungs upon breathing. The lungs are equipped with an immune defense mechanism that involves a wide variety of immunological cells to eliminate these agents. Various types of dendritic cells (DCs) and macrophages (MACs) function as professional antigen-presenting cells (APCs) that engulf pathogens through endocytosis or phagocytosis and degrade proteins derived from them into peptide fragments. During this process, DCs and MACs present the peptides on their major histocompatibility complex class I (MHC-I) or MHC-II protein complex to naïve CD8⁺ or CD4⁺ T cells, respectively. In addition to these cells, recent evidence supports that antigen-specific effector and memory T cells are activated by other lung cells such as endothelial cells, epithelial cells, and monocytes through antigen presentation. In this review, we summarize the molecular mechanisms of antigen presentation by APCs in the lungs and their contribution to immune response.

cGAS-STING and MyD88 Pathways Synergize in Ly6Chi Monocyte to Promote Streptococcus pneumoniae-Induced Late-Stage Lung IFNγ Production

The cyclic GMP–AMP synthase–stimulator of interferon genes (cGAS–STING) pathway senses DNA and induces type I interferon (IFN) production. Whether and how the STING pathway crosstalk to other innate immune pathways during pathogen infection, however, remains unclear. Here, we showed that STING was needed for Streptococcus pneumoniae-induced late, not early, stage of lung IFNγ production. Using knockout mice, IFNγ reporter mice, intracellular cytokine staining, and adoptive cell transfer, we showed that cGAS–STING-dependent lung IFNγ production was independent of type I IFNs. Furthermore, STING expression in monocyte/monocyte-derived cells governed IFNγ production in the lung via the production of IL-12p70. Surprisingly, DNA stimulation alone could not induce IL-12p70 or IFNγ in Ly6C^hi monocyte. The production of IFNγ required the activation by both DNA and heat-killed S. pneumococcus. Accordingly, MyD88^−/− monocyte did not generate IL-12p70 or IFNγ. In summary, the cGAS–STING pathway synergizes with the MyD88 pathway in monocyte to promote late-stage lung IFNγ production during pulmonary pneumococcal infection.

Mucosal vaccines - fortifying the frontiers

Mucosal vaccines offer the potential to trigger robust protective immune responses at the predominant sites of pathogen infection. In principle, the induction of adaptive immunity at mucosal sites, involving secretory antibody responses and tissue-resident T cells, has the capacity to prevent an infection from becoming established in the first place, rather than only curtailing infection and protecting against the development of disease symptoms. Although numerous effective mucosal vaccines are in use, the major advances seen with injectable vaccines (including adjuvanted subunit antigens, RNA and DNA vaccines) have not yet been translated into licensed mucosal vaccines, which currently comprise solely live attenuated and inactivated whole-cell preparations. The identification of safe and effective mucosal adjuvants allied to innovative antigen discovery and delivery strategies is key to advancing mucosal vaccines. Significant progress has been made in resolving the mechanisms that regulate innate and adaptive mucosal immunity and in understanding the crosstalk between mucosal sites, and this provides valuable pointers to inform mucosal adjuvant design. In particular, increased knowledge on mucosal antigen-presenting cells, innate lymphoid cell populations and resident memory cells at mucosal sites highlights attractive targets for vaccine design. Exploiting these insights will allow new vaccine technologies to be leveraged to facilitate rational mucosal vaccine design for pathogens including severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) and for cancer.

In vivo reprogramming of pathogenic lung TNFR2+ cDC2s by IFNβ inhibits HDM-induced asthma

Asthma is a common inflammatory lung disease with no known cure. Previously, we uncovered a lung TNFR2⁺ conventional DC2 subset (cDC2s) that induces regulatory T cells (T_regs) maintaining lung tolerance at steady state but promotes T_H2 response during house dust mite (HDM)-induced asthma. Lung IFNβ is essential for TNFR2⁺ cDC2s-mediated lung tolerance. Here, we showed that exogenous IFNβ reprogrammed T_H2-promoting pathogenic TNFR2⁺ cDC2s back to tolerogenic DCs, alleviating eosinophilic asthma and preventing asthma exacerbation. Mechanistically, inhaled IFNβ, not IFNα, activated ERK2 signaling in pathogenic lung TNFR2⁺ cDC2s, leading to enhanced fatty acid oxidation (FAO) and lung T_reg induction. Last, human IFNβ reprogrammed pathogenic human lung TNFR2⁺ cDC2s from patients with emphysema ex vivo. Thus, we identified an IFNβ-specific ERK2-FAO pathway that might be harnessed for DC therapy.

Monocyte-Derived Dendritic Cells (moDCs) Differentiate into Bcl6+ Mature moDCs to Promote Cyclic di-GMP Vaccine Adjuvant-Induced Memory TH Cells in the Lung

Induction of lung mucosal immune responses is highly desirable for vaccines against respiratory infections. We recently showed that monocyte-derived dendritic cells (moDCs) are responsible for lung IgA induction. However, the dendritic cell subset inducing lung memory T_H cells is unknown. In this study, using conditional knockout mice and adoptive cell transfer, we found that moDCs are essential for lung mucosal responses but are dispensable for systemic vaccine responses. Next, we showed that mucosal adjuvant cyclic di-GMP differentiated lung moDCs into Bcl6⁺ mature moDCs promoting lung memory T_H cells, but they are dispensable for lung IgA production. Mechanistically, soluble TNF mediates the induction of lung Bcl6⁺ moDCs. Our study reveals the functional heterogeneity of lung moDCs during vaccination and paves the way for an moDC-targeting vaccine strategy to enhance immune responses on lung mucosa.

STING and transplantation: can targeting this pathway improve outcomes?

Stimulator of interferon genes (STING) is an innate immune sensor of cytoplasmic dsDNA originating from microorganisms and host cells. STING plays an important role in the regulation of murine graft-versus-host disease (GVHD) after allogeneic hematopoietic stem cell transplantation (allo-HSCT) and may be similarly activated during other transplantation modalities. In this review, we discuss STING in allo-HSCT and its prospective involvement in autologous HSCT (auto-HSCT) and solid organ transplantation (SOT), highlighting its unique role in nonhematopoietic, hematopoietic, and malignant cell types.

Recent advances in understanding the Th1/Th2 effector choice

For over 35 years since Mosmann and Coffman proposed the seminal “type 1 T helper (Th1)/type 2 T helper (Th2)” hypothesis in 1986, the immunological community has appreciated that naïve CD4 T cells need to make important decisions upon their activation, namely to differentiate towards a Th1, Th2, Th17 (interleukin-17-producing T helper), follicular T helper (Tfh), or regulatory T cell (Treg) fate to orchestrate a variety of adaptive immune responses. The major molecular underpinnings of the Th1/Th2 effector fate choice had been initially characterized using excellent reductionist in vitro culture systems, through which the transcription factors T-bet and GATA3 were identified as the master regulators for the differentiation of Th1 and Th2 cells, respectively. However, Th1/Th2 cell differentiation and their cellular heterogeneity are usually determined by a combinatorial expression of multiple transcription factors, particularly in vivo, where dendritic cell (DC) and innate lymphoid cell (ILC) subsets can also influence T helper lineage choices. In addition, inflammatory cytokines that are capable of inducing Th17 cell differentiation are also found to be induced during typical Th1- or Th2-related immune responses, resulting in an alternative differentiation pathway, transiting from a Th17 cell phenotype towards Th1 or Th2 cells. In this review, we will discuss the recent advances in the field, focusing on some new players in the transcriptional network, contributions of DCs and ILCs, and alternative differentiation pathways towards understanding the Th1/Th2 effector choice in vivo.

Dendritic Cell Regulation of T Helper Cells

As the professional antigen-presenting cells of the immune system, dendritic cells (DCs) sense the microenvironment and shape the ensuing adaptive immune response. DCs can induce both immune activation and immune tolerance according to the peripheral cues. Recent work has established that DCs comprise several phenotypically and functionally heterogeneous subsets that differentially regulate T lymphocyte differentiation. This review summarizes both mouse and human DC subset phenotypes, development, diversification, and function. We focus on advances in our understanding of how different DC subsets regulate distinct CD4⁺ T helper (Th) cell differentiation outcomes, including Th1, Th2, Th17, T follicular helper, and T regulatory cells. We review DC subset intrinsic properties, local tissue microenvironments, and other immune cells that together determine Th cell differentiation during homeostasis and inflammation.

Monocyte-derived dendritic cells link localized secretory IgA deficiency to adaptive immune activation in COPD

Although activation of adaptive immunity is a common pathological feature of chronic obstructive pulmonary disease (COPD), particularly during later stages of the disease, the underlying mechanisms are poorly understood. In small airways of COPD patients, we found that localized disruption of the secretory immunoglobulin A (SIgA)-containing mucosal immunobarrier correlated with lymphocyte accumulation in airway walls and development of tertiary lymphoid structures (TLS) around small airways. In SIgA-deficient mice, we observed bacterial invasion into the airway epithelial barrier with lymphocytic infiltration and TLS formation, which correlated with the progression of COPD-like pathology with advanced age. Depletion of either CD4⁺ or CD8⁺ T lymphocytes reduced the severity of emphysema in SIgA-deficient mice, indicating that adaptive immune activation contributes to progressive lung destruction. Further studies revealed that lymphocyte infiltration into the lungs of SIgA-deficient mice was dependent on monocyte-derived dendritic cells (moDCs), which were recruited through a CCR2-dependent mechanism in response to airway bacteria. Consistent with these results, we found that moDCs were increased in lungs of COPD patients, along with CD4⁺ and CD8⁺ effector memory T cells. Together, these data indicate that endogenous bacteria in SIgA-deficient airways orchestrate a persistent and pathologic T lymphocyte response through monocyte recruitment and moDC differentiation.

The Age of Cyclic Dinucleotide Vaccine Adjuvants

As prophylactic vaccine adjuvants for infectious diseases, cyclic dinucleotides (CDNs) induce safe, potent, long-lasting humoral and cellular memory responses in the systemic and mucosal compartments. As therapeutic cancer vaccine adjuvants, CDNs induce potent anti-tumor immunity, including cytotoxic T cells and NK cells activation that achieve durable regression in multiple mouse models of tumors. Clinical trials are ongoing to fulfill the promise of CDNs (ClinicalTrials.gov: NCT02675439, NCT03010176, NCT03172936, and NCT03937141). However, in October 2018, the first clinical data with Merck's CDN MK-1454 showed zero activity as a monotherapy in patients with solid tumors or lymphomas (NCT03010176). Lately, the clinical trial from Aduro's CDN ADU-S100 monotherapy was also disappointing (NCT03172936). The emerging hurdle in CDN vaccine development calls for a timely re-evaluation of our understanding on CDN vaccine adjuvants. Here, we review the status of CDN vaccine adjuvant research, including their superior adjuvant activities, in vivo mode of action, and confounding factors that affect their efficacy in humans. Lastly, we discuss the strategies to overcome the hurdle and advance promising CDN adjuvants in humans.

New MoDC-Targeting TNF Fusion Proteins Enhance Cyclic Di-GMP Vaccine Adjuvanticity in Middle-Aged and Aged Mice

Cyclic dinucleotides (CDNs) are promising vaccine adjuvants inducing balanced, potent humoral, and cellular immune responses. How aging influences CDN efficacy is unclear. We examined the vaccine efficacy of 3',5'-cyclic diguanylic acid (cyclic di-GMP, CDG), the founding member of CDNs, in 1-year-old (middle-aged) and 2-year-old (aged) C57BL/6J mice. We found that 1- and 2-year-old C57BL/6J mice are defective in CDG-induced memory T helper (Th)1 and Th17 responses and high-affinity serum immunoglobulin (Ig)G, mucosal IgA production. Next, we generated two novel tumor necrosis factor (TNF) fusion proteins that target soluble TNF (solTNF) and transmembrane TNF (tmTNF) to monocyte-derived dendritic cells (moDCs) to enhance CDG vaccine efficacy in 1- and 2-year-old mice. The moDC-targeting TNF fusion proteins restored CDG-induced memory Th1, Th17, and high-affinity IgG, IgA responses in the 1- and 2-year-old mice. Together, the data suggested that aging negatively impacts CDG vaccine adjuvanticity. MoDC-targeting TNF fusion proteins enhanced CDG adjuvanticity in the aging mice.

Antigen presentation by dendritic cells and their instruction of CD4+ T helper cell responses

Dendritic cells are powerful antigen-presenting cells that are essential for the priming of T cell responses. In addition to providing T-cell-receptor ligands and co-stimulatory molecules for naive T cell activation and expansion, dendritic cells are thought to also provide signals for the differentiation of CD4+ T cells into effector T cell populations. The mechanisms by which dendritic cells are able to adapt and respond to the great variety of infectious stimuli they are confronted with, and prime an appropriate CD4+ T cell response, are only partly understood. It is known that in the steady-state dendritic cells are highly heterogenous both in phenotype and transcriptional profile, and that this variability is dependent on developmental lineage, maturation stage, and the tissue environment in which dendritic cells are located. Exposure to infectious agents interfaces with this pre-existing heterogeneity by providing ligands for pattern-recognition and toll-like receptors that are variably expressed on different dendritic cell subsets, and elicit production of cytokines and chemokines to support innate cell activation and drive T cell differentiation. Here we review current information on dendritic cell biology, their heterogeneity, and the properties of different dendritic cell subsets. We then consider the signals required for the development of different types of Th immune responses, and the cellular and molecular evidence implicating different subsets of dendritic cells in providing such signals. We outline how dendritic cell subsets tailor their response according to the infectious agent, and how such transcriptional plasticity enables them to drive different types of immune responses.

TBK1 and IKKε Act Redundantly to Mediate STING-Induced NF-κB Responses in Myeloid Cells

Stimulator of Interferon Genes (STING) is a critical component of host innate immune defense but can contribute to chronic autoimmune or autoinflammatory disease. Once activated, the cyclic guanosine monophosphate (GMP)-adenosine monophosphate (AMP) (cGAMP) synthase (cGAS)-STING pathway induces both type I interferon (IFN) expression and nuclear factor-κB (NF-κB)-mediated cytokine production. Currently, these two signaling arms are thought to be mediated by a single upstream kinase, TANK-binding kinase 1 (TBK1). Here, using genetic and pharmacological approaches, we show that TBK1 alone is dispensable for STING-induced NF-κB responses in human and mouse immune cells, as well as in vivo. We further demonstrate that TBK1 acts redundantly with IκB kinase ε (IKKε) to drive NF-κB upon STING activation. Interestingly, we show that activation of IFN regulatory factor 3 (IRF3) is highly dependent on TBK1 kinase activity, whereas NF-κB is significantly less sensitive to TBK1/IKKε kinase inhibition. Our work redefines signaling events downstream of cGAS-STING. Our findings further suggest that cGAS-STING will need to be targeted directly to effectively ameliorate the inflammation underpinning disorders associated with STING hyperactivity.

Selective Loss of Responsiveness to Exogenous but Not Endogenous Cyclic-Dinucleotides in Mice Expressing STING-R231H

Stimulator of interferon genes (STING) plays a central role in innate immune responses to viral and intracellular bacterial infections, and cellular damage. STING is a cytosolic sensor of cyclic dinucleotides (CDNs) including those produced by pathogenic bacteria and those arising endogenously as products of the DNA sensor cGAS (e.g., 2′3′ cGAMP). The two most common alternative allelic variants of STING in humans are STING-R71H-G230A-R293Q (STING-HAQ) and STING-R232H that are found in 20.4% and 13.7–17.6% of the population, respectively. To determine the biologic consequences of these genotypic variations, we generated knock-in mice containing the murine equivalents of each variant and studied their responsiveness to CDNs. Homozygous STING-HAQ (R71H-I229A-R292Q) and STING-R231H mice were found to be unresponsive to all exogenous CDNs tested (ci-di-GMP, ci-di-AMP, 3′3′ cGAMP and Rp,Rp-CDA). Responses of homozygous STING-HAQ mice to endogenous 2′3′ cGAMP was also greatly impaired. However, homozygous STING-R231H mice are fully responsive to 2′3′ cGAMP. Analysis of heterozygous mice revealed reduced responsiveness to exogenous and endogenous CDNs in mice carrying a single copy of STING-HAQ, while STING-R231H heterozygous mice exhibit reduced responsiveness to exogenous but not endogenous CDNs. These findings confirm and extend previous reports by demonstrating differing impact of allelic variation of STING on the ability to sense and respond to exogenous vs. endogenous CDNs. Finally, the STING-R231H variant mouse represents a useful tool with which to examine the relative contributions of STING sensing of exogenous and endogenous CDNs in the context of bacterial infections and CDN-based cancer immunotherapeutics.

Lung IFNAR1hi TNFR2+ cDC2 promotes lung regulatory T cells induction and maintains lung mucosal tolerance at steady state

The lung is a naturally tolerogenic organ. Lung regulatory T cells (T-regs) control lung mucosal tolerance. Here, we identified a lung IFNAR1^hiTNFR2⁺ conventional DC2 (iR2D2) population that induces T-regs in the lung at steady state. Using conditional knockout mice, adoptive cell transfer, receptor blocking antibodies, and TNFR2 agonist, we showed that iR2D2 is a lung microenvironment-adapted dendritic cell population whose residence depends on the constitutive TNFR2 signaling. IFNβ-IFNAR1 signaling in iR2D2 is necessary and sufficient for T-regs induction in the lung. The Epcam⁺CD45^- epithelial cells are the sole lung IFNβ producer at the steady state. Surprisingly, iR2D2 is plastic. In a house dust mite model of asthma, iR2D2 generates lung T_H2 responses. Last, healthy human lungs have a phenotypically similar tolerogenic iR2D2 population, which became pathogenic in lung disease patients. Our findings elucidate lung epithelial cells IFNβ-iR2D2-T-regs axis in controlling lung mucosal tolerance and provide new strategies for therapeutic interventions.

Evaluation of Mucosal and Systemic Vaccine Responses by Cyclic di-GMP (CDG)-adjuvanted Protein Subunit Vaccines

Intranasal administration of vaccine adjuvants directly deliver therapeutic agents to the lungs to induce potent lung mucosal immune responses. Cyclic di-GMP (CDG) is a promising mucosal vaccine adjuvant candidate capable of inducing protective immunity. This protocol describes an in vivo approach to induce and detect mucosal (lung) and systemic (blood and spleen) vaccine adjuvant responses of CDG. This protocol also includes the methods to detect both humoral and cellular immune responses of CDG adjuvant. Last, this protocol can be used to study other cyclic dinucleotides as mucosal vaccine adjuvants.